1. Field of the Invention
The present invention relates to a DC-DC converter which uses a xcex94xcexa3 modulator to limit excessive peak current flowing to an inductor and power switching element by monitoring the current flowing to said inductor with an integrator composed with a xcex94xcexa3 modulator.
2. Description of the Related Art
xcex94xcexa3 modulation is a modulation method in which modulator input signal us integrated, quantization is carried out by comparing the integrated value with a reference voltage, and modulator output signal is fed back to the modulator input. FIG. 3 is a block drawing of a primary xcex94xcexa3 modulator. A DC-DC converter that switches power switching element 8 can be produced using this modulation system.
FIG. 4 shows a step-down chopper DC-DC converter as an example of a method of the prior art that uses xcex94xcexa3 modulation. This DC-DC converter is equipped with smoothing circuit 18, xcex94xcexa3 modulator 6 is connected to the output side of this smoothing circuit 18 via an error amplification circuit 13, and output side of this xcex94xcexa3 modulator 6 is connected to the input of power switching element 8 via a gate driver circuit 7.
A constant voltage output can be obtained by comparing the converter output voltage with a reference voltage, performing xcex94xcexa3 modulation by using the error amplification signal voltage of the compared voltage as the modulator input signal of xcex94xcexa3 modulator 6, switching power switching element 8 according to the output signal of modulator 6, and inputting the above switching output to smoothing circuit 18.
In contrast to DC-DC converters using a pulse width modulation system (to be referred to as PWM), which has typically been used in the past, being characterized by the switching frequency being constant, DC-DC converters using xcex94xcexa3 modulation are synchronized with the sampling frequency given by the xcex94xcexa3 modulator, and the output 1 bit signal is changed so that the error between the modulator input signal voltage and modulator output signal voltage becomes minimal. Consequently, the xcex94xcexa3 modulator input signal voltage changes according to the DC-DC converter output voltage and as a result, this modulation system has the characteristic of the switching frequency changing.
Although attention has been drawn to the advantage of being able to improve the power conversion efficiency of the converter as compared with the PWM system as a result of holding switching loss, which is power loss due to switching, to a low level since the number of switching cycles is reduced in xcex94xcexa3 modulation particularly when the DC-DC converter output current is small, this has the problems described below.
DC-DC converters have the characteristic that, during the time power switching element 8 is off, current flows through rectifier 14 while capacitor 15 discharges, and due to the presence of inductor 16, current decreases gradually instead of sharply, while during the time power switching element 8 is on, current flows to inductor 16 through power switching element 8, capacitor 15 discharges, and due to the presence of inductor 16, current rises gradually, and the current that flows rises the longer the period during which power switching element 8 is on.
In the PWM system, since the switching frequency is constant, although the period during which power switching element 8 is on is nearly constant provided the DC-DC converter output current is constant, in the case of xcex94xcexa3 modulation, since the switching frequency is not constant, there are cases in which a state occurs in which power switching element 8 is on for a long period of time even if DC-DC converter output current is constant, and this state occurs easily particularly when the DC-DC converter output current is large.
Due to the above characteristics, since cases can occur in which the period during which power switching element 8 is on becomes long in xcex94xcexa3 modulation as compared with PWM modulation, there was the problem of the peak current flowing inside the converter becoming large.
Since power loss of power switching element 8 is determined by the product of the on resistance of power switching element 8 and the square of the current flowing to power switching element 8, if the current flowing to power switching element 8 increases, the power loss in power switching element 8 becomes extremely large. Moreover, since the power loss in rectifier 14 is determined by the forward voltage and current that is flowing, and power loss also becomes large when the current flowing to rectifier 14 increases, particularly in cases in which the DC-DC converter output current is large, converters using xcex94xcexa3 modulation had the problem of poor power conversion efficiency as compared with those using PWM modulation due to the large current flowing inside the converter.
That is, in the case of xcex94xcexa3 modulation of the method of the prior art, although there is the advantage of high power conversion efficiency as compared with PWM modulation when the DC-DC converter output current is small, when the DC-DC converter output current is large, cases occur in which the peak current flowing inside the converter becomes large, thereby resulting in the problem of a decrease in power conversion efficiency.
In consideration of the above problems of the prior art, the object of the present invention is to provide a highly efficient DC-DC converter that uses xcex94xcexa3 modulation to limit excessive peak current flowing inside the converter by monitoring the current flowing inside the converter with an integrator located inside a xcex94xcexa3 modulator.
The present invention for achieving the above object is a DC-DC converter that switches a power switching element by inputting an analog input signal or multi-bit digital signal into a xcex94xcexa3 modulator, and conveying that modulated signal to the power switching element via a gate driver circuit, wherein excessive peak current flowing to an inductor is limited by monitoring the current flowing inside the converter with an integrator located inside the xcex94xcexa3 modulator.
The xcex94xcexa3 modulator is composed of at least one adder, integrator and quantizer, has a feedback pathway that feeds back from the output of the quantizer to the input of at least one adder, at least one integrator is connected to the output of the adder, and at least one output of the integrator is connected to the quantizer.
The DC-DC converter is also provided with an adder serving as the input of the xcex94xcexa3 modulator and an integrator connected to the output of that adder, and an analog adder and analog integrator are used in the case of inputting a continuous time signal in the manner of an analog signal to a modulator, while a digital adder and digital integrator are used in the case of inputting a discrete time signal in the manner of a multi-bit digital signal to a modulator.
The quantizer performs sampling on the discrete time signal, while the output of this quantizer and the output of a current control circuit, to which is connected the output of the above integrator and the output of an error amplification circuit, are connected to the input of an AND circuit that determines the final pulse width, and the output signal of this AND circuit is connected to the input of a gate driver circuit that supplies current and voltage sufficient for driving the power switching element.
The output of the above gate driver circuit is connected to the input of a power switching element that switches the input voltage of the DC-DC converter, while the output of the power switching element is connected to a smoothing circuit, and the output of this smoothing circuit becomes the output of the DC-DC converter.
The above feedback pathway in the xcex94xcexa3 modulator is equipped with an attenuator that conforms to output signal of the quantizer to the input signal level of the xcex94xcexa3 modulator. However, an attenuator having an attenuation factor of 0 is also considered to be included in the case the signal level of the quantizer output and the signal level of the xcex94xcexa3 modulator input are already conforming at the outset, and an attenuator is not required.
The present invention has a constitution such that, the output of the above integrator and the output of the above error amplifier are compared by the above current control circuit, an excessive peak signal is generated with the output of the above integrator, and when that excessive peak signal has reached the output signal level of the above error amplifier, a pulse signal is output from the current control circuit, and a signal that switches off the power switching element is applied to the power switching element through a gate driver circuit by the above AND circuit after having received the pulse signal output from the current control circuit.
When large power from a load connected to the DC-DC converter increases and the converter output current increases to a certain extent, although the current flowing to the inductor within the smoothing circuit is always at 0 A or more, and its waveform is a chopping wave that repeatedly increases and decreases according to the on and off state of the power switching element, even though there are cases in which the output voltage of the integrator within the xcex94xcexa3 modulator is in the form of a similar chopping wave, and is inverted due to the circuit configuration of the DC-DC converter, since the waveform of the current flowing to the inductor and the waveform of the output voltage of the integrator are analogous, an effect can be obtained that is similar to monitoring the current flowing to the inductor, namely the current flowing within the DC-DC converter, by monitoring the output of the integrator.
Thus, since switching off the power switching element by detecting the peak output voltage of the integrator is equivalent to switching off the power switching element by detecting the peak current flowing within the DC-DC converter, the present invention makes it possible to suppress excessive peak current flowing within the DC-DC converter, and enables the realization of a highly efficient DC-DC converter.
In a DC-DC converter using a xcex94xcexa3 modulator, by monitoring the current flowing to an inductor with an integrator composed within the xcex94xcexa3 modulator, excessive peak current flowing within the converter can be limited, making it possible to provide a DC-DC converter capable of highly efficient, stable control without requiring a detection resistor.